The following discussion deals with starter tubes and sealing arrangements for optical fiber pre-forms, but it is to be understood that principles of the present invention are applicable to other, different applications involving, generally, chemical delivery systems wherein the chemicals are in gaseous or vapor form.
Optical fiber of the type used to carry optical signals is fabricated typically by heating and drawing a portion of an optical pre-form comprising a refractive core surrounded by a protective glass cladding. Presently, there are several known processes for fabricating pre-forms. The modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027 issued in the names of J. B. MacChesney et al. on Aug. 12, 1980 and assigned to Bell Telephone Laboratories, Inc. has been found to be one of the most useful because the process enables large scale production of pre-forms which yield very low loss optical fiber.
During the fabrication of pre-forms by the MCVD process, reactant-containing gases, such as SiCL.sub.4 are passed through a rotating substrate tube which is made of silica glass. A torch heats the tube from the outside as the precursor gases are passed therethrough, causing deposition of submicron-sized glass particles on the inside surface of the tube. The torch is moved along the longitudinal axis of the tube in a plurality of passes to build up layer upon layer of glass to provide a pre-form tube. Once a sufficient number of layers have been deposited, the pre-form tube is then heated to cause it to be collapsed to yield a pre-form or pre-form rod as it is often called. The delivery system of the reactant gases to the starter tube interior is generally through a fixed metallic hollow tube connected to the source or sources of the gases. It is necessary that the space between the exterior surface of the delivery tube and the interior surface of the glass starter tube be sealed so that the critical gases do not leak out of the starter tube.
In the current method of manufacture, the apparatus which ensures sealed delivery of the deposition chemicals in the gases is a combination of a rotary union element, a structure for holding and sealing the starter tube, and a secondary face seal assembly for routing of purge gases through the structure. This is a complex apparatus that requires frequent maintenance. Existing systems also have the disadvantage of having inherently larger cavities for the accumulation of dead zones of flow, and a tendency to create particle contamination from the rotary union and face seal system. Inasmuch as the chemical delivery system is stationary, the current means of achieving delivery is via the rotary union, featuring a transition of the chemicals from a stationary pipe to a rotary pipe or to the inside of a supply coupling. The chemicals being delivered are at a pressure greater than atmospheric, and the face seal properties are the only restriction to the release of the chemicals to the atmosphere. The rotary union and secondary face seals generate a large quantity of particles from wear, and contribute to the contamination of the coupling. As the seals wear down, excessive leakage occurs both to the atmosphere and into the product (starter tube), resulting in lost product and requiring system maintenance. The complexity of the components involved requires skilled maintenance being performed using requalification through test of the system. Both material and labor costs are, consequently, high.
Present day techniques and apparatus provide a buffer zone of gas/liquid between the chemical being delivered into the atmosphere. The buffer zone provides for a controlled environment, which may be regulated to pressures higher or lower than the surrounding chemical being delivered and higher/lower than the atmosphere. Selection of this buffer zone pressure and constituent gas/liquid in this zone controls the leakage directions and the leakage constituents across the seals. In a typical buffered sealing system, the buffered volume is achieved most simply by having a single internal seal and a single external seal connected via a housing which is fixed relative to the rotating member to be sealed. The introduction passage and the removal passage are then connected through the housing to the buffer volume.
In Mueller patent application Ser. No. 09/497,042 filed Feb. 2, 2000, the disclosure of which is incorporated herein by reference, there is shown a sealing system that eliminates many drawbacks characteristic of prior art delivery systems, as enumerated in that application, such as, for example, the rotary union, by internally sealing the starter tube by means of a self tightening seal and mounting arrangement therefor. The basis of such the arrangement of that application requires a constant rotational capability of the mounting hub for the self tightening feature, which conflicts with the need for fixed connections to the buffer zone of the introduction and removal lines. Inasmuch as supply lines cannot be connected directly to the components of the Mueller arrangement, the advantages to be gained through the use of disposable, self-tightening seals would appear to be unrealizable.
In Mueller patent application Ser. No. 09/383,786 filed Aug. 8, 1999 now U.S. Pat. No. 6,210,487 issued Apr. 3, 2001 there is shown a dual internal seal system which eliminates the aforementioned conflict. The system shown in that application is for use with a stationary chemical supply apparatus including a stationary delivery tube having a first end upon which is mounted a supply coupling having an extension inserted into the starter tube of an optical pre-form starter tube. A primary seal is mounted to the first end of the extension tube and functions to seal the interior of the starter tube from the atmosphere or ambient environment, thus preventing deleterious leakage loss of the chemical vapor introduced therein through the extension tube. A secondary seal, also mounted on the delivery tube, is spaced from the primary seal to form a buffer zone between the two seals. A supply coupling receives oxygen, for example, from a controllable pressure source, and introduces it into the buffer zone between the two seals. The supply coupling also is connected to a vacuum source for exhausting the gas mixture from the buffer zone, thereby creating a continuous gas flow.
As discussed in the aforementioned Mueller application Ser. No. 09/383,780, the mounting means for the seals may take any of a number of forms, however, a preferred arrangement is a threaded end on the insert and interior threads in the seal nut. Also as explained in the Mueller application, the direction of the threads, i.e., left hand or right hand, should be the same as the direction of rotation of the starter tube in order to apply a tightening torque to the seal and to the seal nut. The same constraint applies to other types of fastening or mountings which require a twisting or turning action, such as, for example, bayonet mounts. The primary and secondary conical seals are longitudinally spaced and, in use, form a buffer zone that is sealed by the primary seal from the starter tube interior, and, by the secondary seal from the atmosphere.
In use, the oxygen or other buffer gas or liquid is introduced under regulated pressure into the supply coupling housing and distributed by the first group of channels to the buffer zone. The buffer zone gases, which may include some leakage from the starter tube, are exhausted through the second group of channels to the vacuum apparatus coupling. Thus, there is a constant circulation of gaseous/liquid material through the buffer zone which, as discussed hereinbefore, makes possible a controlled environment, control over leakage, cooling of the apparatus, and removal of contaminants or other unwanted materials. These desiderata are realized in a relatively simple apparatus having replaceable seals without the necessity of disturbing or piercing the seals.
The just described dual sealed buffer zone arrangement has been found to yield excellent results. However, it represents a new system in which several components used in formation of preforms are modified to accommodate the new components. Inasmuch as there exist, today, numerous gas delivery systems already in place, it would be highly desirable to retrofit them with minimum modification, to produce a dual sealed buffer zone system based upon principles described in the foregoing.